The present invention relates to a polypropylene composition and more particularly to a polypropylene composition composed mainly of a crystalline ethylene-propylene block copolymer, which has excellent moldability and can provide a molded article superior particularly in impact resistance, coatability and appearance.
Crystalline polypropylenes are in use in various molded articles in which physical properties such as rigidity, resistance to heat deformation and the like are required. However, having insufficient low temperature properties and being a non-polar highly-crystalline polymer, crystalline polypropylenes have poor impact resistance and poor coatability and cannot be used in applications in which rigidity, resistance to heat deformation, impact resistance and coatability are required together.
In order to improve the impact resistance and coatability of crystalline polypropylenes, there have been proposed, for example, a polypropylene composition which is a blend of a crystalline ethylene-propylene block copolymer as a crystalline polypropylene with an amorphous ethylene-propylene copolymer, and a polypropylene composition which is a blend of crystalline ethylene-propylene block copolymer, an amorphous ethylene-propylene copolymer and various polyethylenes.
In these polypropylene compositions, however, the rigidity, resistance to heat deformation, etc. inherently possessed by crystalline ethylene-propylene block copolymers are reduced.
Japanese Patent Application Kokai (Laid-Open) No.64257/1978 discloses a resin composition with improved low temperature impact resistance, comprising a crystalline ethylene-propylene block copolymer, an amorphous ethyleneolefin copolymer and talc. The document describes that the resin composition, as compared with conventional crystalline ethylene-propylene block copolymers, is significantly improved in low temperature impact resistance, rigidity and coatability.
The compositions described in Examples of the above patent document use, as polymer components, a crystalline ethylene-propylene block copolymer having a melt flow index of 1.5, and an ethylene-propylene-nonconjugated diene copolymer having a melt flow index of 0.4 or an amorphous ethylene-propylene copolymer having a melt flow index of 0.8. All of these polymers have a very high molecular weight. Resultantly, the above compositions are fully satisfactory with respect to the purpose of obtaining a polypropylene molded article superior in rigidity, heat deformation resistance and impact strength, but have drawbacks in that the molded articles produced therefrom have flow marks on the surface, resultantly have poor appearance, have insufficient coatability, and thus have a low commercial value. Consequently, the molded articles obtained from the compositions are unsuitable for use in applications (e.g. parts for automobiles, particularly bumper) where mechanical properties and appearance are required, although the articles can find limited applications where appearance can be neglected.
Polypropylene compositions with improved impact resistance are proposed also in Japanese Patent Application Kokai (Laid-Open) Nos. 136735/1976, 22552/1978, 64256/1978, 21494/1980, 55952/1982, 159841/1982, 159842/1982, 207630/1982, 177038/1982, 111846/1983, 168648/1983, etc. These polypropylene compositions, however, are inferior in moldability and accordingly give molded articles with poor appearance.